1. Field of the Invention
This invention relates to fluid-absorbing structures for use in items such as disposable diapers and the like, which are biodegradable and flushable.
2. Description of the Related Art
While the fluid-absorbing structures of the invention are illustrated and described as applicable to use in disposable diapers, it is recognized that the fluid-absorbing structures may be utilized in a number of different products, for example, sanitary napkins, bed liners, bed pan liners, absorbent cloths, and the like.
While there are a number of different disposable diapers on the market, several generalizations about the characteristics of a conventional disposable diaper can be described. A conventional disposable diaper comprises a top sheet which serves as an inner porous liner, a fluid absorbent center ply and a liquid impermeable bottom cover. The top sheet is typically formed from a nonbiodegradable nonwoven fabric produced from a type of synthetic fiber, such as polyester or polypropylene fibers, and typically has a weight of approximately 0.7 oz/yd.sup.2. The fluid absorbent center ply of a conventional disposable diaper is predominantly a cellulosic absorbent fluff produced from low density softwood fluff fibers or from blends of softwood and hardwood fluff fibers. Frequently a nonbiodegradable superabsorbent hydrocolloid material is blended into the fluff fibers of the center ply to improve the wicking rate and fluid-holding capacity of the cellulosic fluff in the center ply. Typically the nonbiodegradable superabsorbent material is a polyacrylic acid derivative. The bottom cover of a conventional disposable diaper comprises a monolithic nonbiodegradable film manufactured from polyolefins, such as polyethylene, ranging in thickness of approximately 1 to 1.2 mils.
Prior art patents address virtually all aspects of the disposable diaper. The patent of Bernardin (U.S. Pat. No. 5,009,650), for example, describes an absorbent batt for use in disposable diapers having at least two layers of cellulosic fibers which have different densities or pore sizes, and optionally a superabsorbent material layer. The patent of Pratt (U.S. Pat. No. 5,026,363) describes a disposable diaper having a disassembly or separation means by which the diaper layers are separated and made flushable.
Conventional disposable diapers, however, are not without problems. First, conventional disposable diapers are not biodegradable. While claims of biodegradability are made about some diapers, often the diapers are not fully biodegradable because each and every component is not subject to biodegradation. Second, diapers which utilize biodegradable materials often do not perform adequately in controlling and retaining the fluids introduced into them. For example, diapers having layers comprised of biodegradable films, such as those manufactured from polyvinyl alcohol, are problematic in that the films are water soluble and do not adequately act as fluid barriers to aqueous challenges.
Conventional disposable diapers are typically said to be nonbiodegradable due to the presence of the various non-biodegradable components: the synthetic materials used in the top sheets, the superabsorbents used in the center plys, and the bottom covers comprised of polyolefin films. Many inventors have attempted to provide a biodegradable diaper which both absorbs fluids adequately and is biodegradable. A first problem in creating alternatives to nonbiodegradable disposable diapers, however, is in defining "biodegradable".
Controversy surrounds the definition of "biodegradable". A material is frequently said to be "biodegradable" if when exposed to certain microorganisms, the material is converted into "compost", a process called "composting". The term "biodegradable" has been applied to a converted material even if only a small percentage of the material is actually converted. A material is only fully biodegraded when the microorganisms have depleted all food value from the material. Once all the food value has been depleted from the material, the natural mineral dirt called "humus" remains. Materials which are said to be nonbiodegradable are incapable of being comsumed by microorganisms. As used in this description, the term "biodegradable" is meant to refer to a material which is capable of being entirely consumed by microorganisms to form humus within a relatively short period, for example, six (6) weeks. Additionally, in this description, the terms "biodegradable" and "compostable" are used interchangeably.
The producers of disposable diapers are actively pursuing the development of disposable diapers which are more compatible with the environment by trying to develop biodegradable and flushable diapers. Criticism has been lodged against the use of disposable diapers by environmental protection groups who claim that disposable diapers do not degrade in the landfills and essentially remain unchanged in the ground forever. Several states have threatened to prohibit the sale of disposable diapers; some states have passed legislation to tax disposable diapers in order to cover the cost of disposing of the diapers.
The disposable diaper has, nevertheless, been a boon to the modern parent. The convenience of using disposable diapers as an alternative to reusable (e.g., cloth) diapers has led to an almost total conversion to disposable diapers. In addition to the convenience, disposable diapers have virtually eliminated the discomfort of diaper rash, a skin irritation associated with the use of reusable diapers. A recent article addresses the issue of disposable versus reusable diapers (Consumer Reports, August 1991, pp. 551-556).
A second problem in developing an improved disposable diaper is to create a biodegradable disposable diaper which is adequate as a fluid barrier or fluid retainer. In this context, a fluid barrier is a layer of a substance which is impervious to fluid leakage, such as a polyolefin film in a diaper bottom cover. A fluid retainer is a layer of a substance which holds fluid within it, such as a center ply containing a superabsorbent. The polyolefin bottom cover of the currently available disposable diaper is an excellent fluid barrier, but it is not nonbiodegradable. The bottom cover of a disposable diaper must act as a fluid barrier to prevent fluid leakage from the center ply to the outside of the diaper.
One proposed solution to the problem of poor degradability of the disposable diaper has focused on modifying the diaper bottom cover by the addition of a biodegradable component, such as starch, to the polyolefin film which contains high molecular weight segments that do not biodegrade. It is believed that in biodegradation, when the microorganisms attack the biodegradable starch component, the polyolefin film will be broken down into smaller segments which are more compatible with the environment. Because the addition of starch tends to weaken the film, however, a thicker film is utilized in the diaper bottom cover. Therefore, the net amount of diaper bottom cover which is actually biodegraded is slight.
Because portions of conventional disposable diapers are nonbiodegradable, i.e., will not be consumed by microorganisms to form humus, other degradation methods for degrading conventional diapers are sought. However, other degradation methods, such as photodegradation, thermal degradation and hydrolytic degradation, are also inadequate methods because it is becoming more difficult to expose waste products, especially those in landfills, to the proper degradation conditions, such as light, heat and water.
The invention recognizes that the geotextile industry has for many years used sodium bentonite in liners which form fluid barriers in land fills or in ponds to prevent fluid leakage from the landfills or ponds to the surrounding ground areas. Of course, the objective of these geotextile liners, unlike that for a diaper, is to engineer as durable a system as possible and to purposely avoid biodegradation. These geotextile liner products are designed to remain virtually unchanged for more than 30 years as required by recent EPA legislation. The liners are heavy systems weighing approximately one pound per square foot and supposedly can resist hydrostatic pressures equivalent to 150-foot head pressure of water. Examples of such liners are: the CLAYMAX.RTM. liner of the James Clem Corporation of Chicago, Ill.; a liner made by Colloid Environmental Technologies Company of Arlington Heights, Ill.; and a liner made by Gundle Company of Houston, Tex.
The invention recognizes that a conventional disposable diaper could be made more biodegradable in a practical sense if the fluid barrier section could be made of biodegradable substrates which together create an outer cover which prevents fluid leakage, is relatively strong, and provides a pleasant fabric feel. As will become apparent from later description, the invention is based upon the discovery that sodium bentonite, even though never before used, so far as is known, in a disposable diaper or similar fluid-absorbing structure, can be incorporated as a fluid barrier in the outer cover of a disposable diaper when such outer cover is made of biodegradable substrates, thereby making an outer cover which resists fluid leakage, and most importantly, is a biodegradable diaper component. The use of a biodegradable substrate creates a diaper outer cover which has a pleasant fabric texture, rather than the harsh synthetic texture of polyolefin films used in a conventional disposable diaper.
A typical geotextile liner has a layer of sodium bentonite sandwiched between two layers of nonbiodegradable fabric layers, such as heavy-weight polyethylene or polypropylene, which layers are bonded with a degradation-resistant binder or by needlepunching. The liner may be applied to the bottom of a cavity, for example, a landfill or pond. When the geotextile liner becomes wet, the sodium bentonite in the liner becomes gelatinous and swells to approximately twelve times its original volume. The swelled sodium bentonite in the geotextile liner aids in sealing the cavity.
Sodium bentonite has been found to operate as a fluid barrier in the presence of salt contaminants (product literature of Colloid Environmental Technologies Company of Arlington Heights, Ill.), in blends with sand (described by J. Hocks, Journal of Waste Management Research, 5(2), pp. 93-105, 1987), and when blended with vermiculite (Chemical Abstracts, CA78:P75483d). Sodium bentonite is also a popular gellant in the oil well drilling industry where it is used to seal walls of oil wells during the drilling process (American Petroleum Institute, Specification 13A).
Sodium bentonite is not known to be harmful to humans or to the environment, as indicated by its use in an antiperspirant formulation, described in U.S. Pat. No. 4,840,786 of Johnson and Baker. Repeated inhalations of sodium bentonite dust in high concentrations may affect the lungs; however, no adverse effects on skin contacts with sodium bentonite have been reported so far as applicant is aware. Sodium bentonite is thus recognized by the invention as being an inorganic mineral which can be introduced into the environment through disposal processes without harming the environment.
Sodium bentonite, an inorganic clay-type mineral, has been assigned Chemical Abstract identification number CAS #1302-78-9. Sodium bentonite is a powder having a pale-buff or grayish tint and a slightly earthy taste. The density of sodium bentonite is approximately 2.5 g/cm.sup.3. Sodium bentonite is insoluble in water and in organic solvents. Sodium bentonite swells to approximately twelve times its dry volume when wet with an aqueous fluid. It is a member of the silicate family and is frequently present in volcanic minerals. Sodium bentonite is available under a wide range of trade names including: ALBAGEL, PREMIUM USP 4444, BENTONITE L, ACCOFLOC 352, AQUAGEL, PANTHER CREEK BENTONITE, SOUTHERN BENTONITE, ASKANGEL, VOLCLAY BENTONITE BC, VOLCLAY, WILKINITE, BAROCO, BULGARBEN BA, COSINTAM 403, BENTONITE MAGMA, FULBENT 570, HMP 20, K43, MINERAL COLLOID 101, B-236, and ACCO 2585.